Frequency error estimation

ABSTRACT

There is disclosed a technique for generating an improved estimate of the frequency error in a received signal, and more particularly the application of such a technique in the equalization circuitry of a wireless network element.

FIELD OF THE INVENTION

The present invention relates to correction of the frequency shift in areceived signal of a communication system, and particularly but notexclusively in a wireless communication system.

BACKGROUND OF THE INVENTION

In known wireless communication systems, an equalizer is used in thereceiver in order to equalize the received signal prior to furtherprocessing in the receiver.

However in a mobile communication system, a mobile may be moving quicklyor have a carrier offset, such that the signal received in the receiverhas a frequency shift.

This frequency shift needs to be removed in the receiver for thereceived signal to be properly processed. More particularly, thefrequency shift should be removed prior to equalization of the receivedsignal.

Existing wireless communication systems utilize techniques for removingthis frequency shift. In one example, a Kalman filter based recursiveestimator is used to remove the frequency shift and equalize thereceived signal.

The problem is to be able to receive a signal from a fast moving mobileor from a mobile where there is a carrier offset. The received signalhas a frequency shift, which should be corrected.

The frequency error is usually estimated from the samples of thereceived samples. This estimate is typically worse when the channelquality is poor and gets better as the channel quality improves. Whenthere is no frequency shift and the channel conditions are near thesensitivity level of a receiver, a frequency correction algorithm shouldnot degrade the performance of the receiver. The problem of frequencyerror correction has become a potential problem in the case of EDGEwireless systems in particular, and as such improvements are desired.

A Maximum-Likelihood method for frequency estimation is defined in M.Luise and R. Reggiannini, “Carrier Frequency Recovery in All-DigitalModems for Burst-Mode Transmissions, IEEE Trans. On Comm.,February/March/April 1995. Also Australian Patent No. AU 664626describes a system where the Doppler correction is based to change inTOA (Time of arrival).

It is an aim of the present invention to provide an improved techniquefor estimating and removing the frequency shift in a received signal ina communication system such as a wireless communication system.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofestimating the frequency error in a received signal, comprising: a)Receiving the signal at time t; b) Removing from said signal an estimateof the frequency error in said signal, thereby generating a frequencycorrected received signal at time t; c) Equalizing said received signal,wherein the equalizing step introduces a delay of n samples, such thatan equalized output is generated at time t−n; d) Generating an estimateof a first component of the frequency error in the received signal attime t−n based on the equalized output; e) Recalculating the first andsecond components in dependence on the frequency corrected receivedsignal at time t; f) In dependence on the recalculated first and secondvalues, estimating values of the first and second components of thefrequency error for times t+n and t+1 respectively; and g) Wherein thefirst component of the estimated frequency error for time t+n is used togenerate a frequency corrected signal for a signal received at time t+n.

The step of generating an estimate of the first component andrecalculating the first component (steps d and e) may use the acquireddecision vector:β={circumflex over (φ)}(t|t−1)−α(t|t−n){circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T)x(t))

The step of estimating a value of the first component (step h) may usethe predictions:α(t+n|t)={circumflex over (φ)}(t|t){circumflex over (φ)}(t+1|t)={circumflex over (φ)}(t|t)

${P\left( {t + 1} \right)} = {\left( {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}} \right)^{- 1} + {{E\left( {{u(t)}}^{2} \right)}.}}$

The step of removing from said signal an estimate of the frequency errorin the received signal at time t+n may use the prediction:y _(r)(t+n)=y(t+n)e ^(−jα(t+n|t)(t+n))

The method may further comprise generating an estimate of a secondcomponent of the error in the received signal based on an equalizedoutput at time t−1, wherein the step of recalculating further includesrecalculating the second component, and the step of the second componentis estimated for time t+1 in dependence on said recalculated value.

The step of generating an estimate of the first component andrecalculating the first component (steps d and e) may use the acquireddecision vector:{circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−h ^(T) x(t))

The step of generating an estimate of the first component andrecalculating the first component (steps d and e) may use the acquireddecision vector:

${P^{- 1}\left( {t + 1} \right)} = {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}}${circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+P(t+1)δ_(w) ² x^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T) x(t))

when u(t)=0.

The step of removing an estimate of the frequency error in a receivedsignal may comprise rotating the received signal with a prediction ofsaid error.

According to a further aspect the present invention further providesequalization circuitry comprising: rotation means for rotating areceived signal at time t with a prediction of the frequency error inthe received signal, thereby generating a frequency corrected receivedsignal at time t; an equalizer having a delay of n samples and forequalizing the frequency corrected received signal at time t, and forgenerating an equalized received signal at time t+n; a frequency errorcalculator for receiving the equalized received signal at time t+n andthe corrected received signal at time t, and for generating a predictionof the frequency error in a received signal received at time t+n,wherein the frequency error calculator generates an estimate of s firstcomponent of the frequency error in the received signal at time t−nbased on the equalizer output, recalculates this first component independence on the frequency corrected received signal at time t, and independence on such recalculated value estimates a value of the firstcomponent for frequency error at time t+n, wherein this estimate is usedby the rotation means to generate a frequency corrected signal for asignal received at time t+n.

The frequency error calculator further generates an estimate of a secondcomponent of the frequency error in the received signal based on anoutput of the equalizer at time t−1, and wherein said second componentis recalculated and a in dependence thereon a value of the secondcomponent of the frequency error for time t+1 is estimated.

An element of a mobile communication system may include suchequalization circuitry. In particular, a base transceiver station mayinclude such circuitry, or implement the defined method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, an example implementation of the present inventionwill be described with reference to a particular, non-limiting example.The invention is described herein with reference to an example of areceiver in a wireless communication system, and more particularly anEDGE system. However the invention is more generally applicable, and maybe utilized in any communication system in which there is a need toremove a frequency shift in an equalizer.

FIG. 1 illustrates a block diagram of a frequency equalizer inaccordance with a preferred embodiment of the present invention, for anEDGE application. The block diagram includes a pre-filter 10, a mixer16, an equalizer 20, and a frequency error estimator 18.

A received sample to be equalized at time t is provided on line 12 tothe input of the pre-filter 10. The received, filtered sample is thenprovided on line 14 at the output of the pre-filter 10, as representedby sample y(t).

The received, filtered sample at time t, y(t), on line 14 is provided asa first input to the mixer 16. The second input to the mixer isprovided, as discussed further hereinafter, by the output of thefrequency error estimator 18 on line 24. The generation of the signal online 24 by the frequency error estimator 18 is discussed furtherhereinbelow. As is discussed further hereinbelow, for a received sampleat time t on line 14, the sample on line 24 represents the frequencyerror in the received sample at time (t−n), where n represents aprocessing delay in the equalizer.

The output of the mixer on line 22 forms the sole input to the equalizer20 and a first input to the frequency error estimator 18. As will befurther discussed hereinbelow, the output of the mixer on line 22,y_(r)(t), represents an estimate of the received signal with anyfrequency error removed. However as will be apparent from the precedingparagraph, this estimate is a best estimate of the frequency error inthe signal at time t based on an error estimation which was performed attime (t−n).

The equalizer 20, in accordance with conventional techniques, generatestwo output signals. A first output signal on line 28 represents theactual decisions made by the equalizer, and forms the output of theequalizer block. This output typically contains soft information of thereceived data, which soft information contains the decision about thevalue of the signal and in addition some quality information about thecertainty of the decisions (Max-Log-MAP). This operation of theequalizer will be familiar to one skilled in the art.

In some cases, such as in a Viterbi decoder, the output on line 28 isonly made available when a whole received block has been processed. Insuch a case a second output of the equalizer, as represented by line 26,is provided. This second output on line 26 provides ‘tentative’decisions for the frequency error estimator block 18.

This second output on line 25 provides ‘tentative’ decisions for thefrequency error estimator block 18.

Thus the second output signal of the equalizer, on line 26, forms asecond input to the frequency error estimator block 18. It should benoted, however, that the example of FIG. 1 assumed an implementationsuch as a Viterbi decoder. If however the equalizer did not have asecond output for feeding back values, then the input to the frequencyerror estimator 18 may be taken directly from the equalizer output 28.

The frequency error estimator generates an output on line 24 inaccordance with the present invention, as further described hereinafter,in dependence on the inputs on lines 22 and 26.

The received signal on line 12, and consequently the filtered, receivedsignal on line 14, contains a frequency shift. This received frequencyshift needs to be corrected for the correct processing of the receivedsignal.

As mentioned hereinabove, the second input of the mixer on line 24,represents the best estimate of the frequency error in the receivedsample y(t), and as such is the frequency error which is to be removedfrom the received, filtered sample at the first input of the mixer 16.

The output of the mixer 16 on line 22 represents an adjusted version ofthe received sample. More particularly the signal on line 22 representsthe received, filtered signal on line 14, with the estimated frequencyshift removed there from.

This corrected sample y_(r)(t) on line 22 is provided to the equalizer20. The equalizer 20 equalizes the samples on line 22 in a known manner,to provide an equalizer output on line 28 for further processingelsewhere in the receiver of which the equalizer block of FIG. 1 forms apart.

The second output of the equalizer 26 are decisions drawn as separatedecisions from the actual decisions on the first output 28 of theequalizer.

Depending on the actual decisions at the first output 28, the decisionson the output 26 may actually be the same, for example if the actualdecisions on line 28 are Max-Log MAP decisions. Thus the decision outputgives the values of the actually transmitted data, which is the functionof the equalizer block.

The frequency error estimator then processes the signals received on therespective lines 22 and 26 to generate the estimate of the frequencyoffset in the received signal for input to the mixer on line 24.

It should be noted that the equalizer block of FIG. 1 would function toequalize the received signals without any frequency error estimator.However, the quality of the decisions generated by such an equalizerblock would be inferior.

In accordance with the present invention, the frequency error estimator18 is adapted in order to provide improved estimates on line 24 for usein removing the frequency error from the received signal.

At time t the frequency error estimator receives the corrected receivedsignal at time t, y_(r)(t), and the decisions about the received signalat time (t−n) from the equalizer. The frequency error estimator thenuses those values to provide an estimate of the frequency error in thereceived signal at time (t+n). Thus the error calculated by thefrequency error estimator at time t, is used for processing the receivedsignal received at time (t+n). Thus, in FIG. 1, there is shown that theerror signal applied to the mixer 16 at the time t corresponds to anerror calculated at time (t−n).

It should be noted that the equalizer block has a processing delay suchthat it provides the estimates relating to the samples received at atime t after a delay n. n represents the number of samples in theprocessing delay. Thus the frequency error estimates performed by thefrequency error estimator are estimates for the future. Therefore theactual output of the equalizer 20 at the time at which the receivedsample y(t) is received is x(t−n).

Thus at a time t, relying purely on the completed processing of theequalizer 20, only the frequency error estimate at time t−n isavailable.

However, the frequency error estimator operates better if as manysamples as are possibly available are used in the error estimating step.For this reason, in accordance with the present invention, the frequencyerror estimator additionally keeps track of all for all samples up totime t−1. As such, and referring to FIG. 1, it can be seen that thefrequency error estimator receives the actual output of the equalizer 20x(t−n), as well as the most recent ‘intermediate’ result, x(t−1).

The frequency error estimator operates on these two values to producetwo corresponding estimates of the frequency error in the signalreceived at time t.

The two frequency error estimates are generated using equations asdefined in the mathematical analysis given hereinbelow.

Thus, the frequency error estimate generated by the frequency errorestimator has two components. The first component, α, is the most recenterror estimate based on the results available at time t−n, and thesecond component, β, is based on all available sample results at timet−1. It should be clear that the second component includes partprocessed results, as it includes in its estimates all samples from t−nto t−1.

As discussed further hereinbelow, the second component β is used to keeptrack of the best possible frequency error estimate, and the firstcomponent α is the information that is actually output by the frequencyerror estimator to the mixer to correct the received signal.

Thus, in a first ‘prediction’ step, the frequency error estimatorgenerates two predictions of the frequency error in the received signal:i.e. the first α and second β components. As a result, at a given time ttwo estimates of the frequency error are available.

The frequency error estimator also receives the adjusted (or frequencycorrected) sample at time t, y_(r)(t). After receiving this sample, thefrequency error estimator can recalculate the best possible frequencyerror estimate at time t. That is, the first α and second β componentsof the frequency error information are corrected.

Thus, in a ‘correction’ step, corrected first α and second β errorcomponents are generated.

After this correction step, the best error estimate for the time t isavailable. However, the samples related to that time have already beenreceived and processed by the equalizer 20, and used to generate thecorrected components α,β as discussed above. As such, these correctedcomponent values cannot be used for removing the frequency error at timet.

The frequency error estimator therefore makes a further estimate of thefrequency error estimate for each component. For the first component α,the estimate is made for a time t+n, and for the second component β theestimate is made for a time t+1. That is, estimates are made for thetime at which the next results are available.

The first component α(t+n) provides the output of the frequency errorestimator, and as such provides the frequency error estimate to beremoved from the received signal y(t+n). The second component β(t+1) isused for the optimality of the Kalman process within the Kalman filter,and is used only within the frequency error estimator, and not outputtherefrom. The use of this second component β ensures that the frequencyerror estimates always use the latest data estimates from the equalizer.

The invention may be further understood with reference to the followingmathematical analysis of the operation of the equalizer block shown inFIG. 1.

The problem of frequency estimation can be written as the followingwell-known equation:y(t)=h ^(T) x(t)e ^(jφt) +v(t)where h^(T)x(t) is a convolution between data and channel impulseresponse, y is a received sample and v is a noise component. In thefrequency estimation the angle φ is to be estimated.

This problem is non-linear and therefore an optimal estimator isdifficult to obtain.

The above equation can be partitioned into linear and non-linear partsby setting:φ=α+β, theny(t)=h ^(T) x(t)e ^(jφt) +v(t)=h ^(T) x(t)e ^(j(α+β)t) +v(t)=h ^(T)x(t)e ^(jαt)(1+jβt)+v(t)  /1/

A linearisation step has been used assuming e^(jβt)≈(1+jβt). Assumingwhite noise both sides can be multiplied by e^(−jαt) and now a “partlylinear” model can be written asy(t)e ^(−jαt) −h ^(T) x(t)=jβth ^(T) x(t)+w(t)  /2/

-   -   where φ=α+β,

where the frequency error φ is to be estimated. As such the components αand β are to be estimated. This mathematical model defines the basis forthe frequency error estimator according to the invention which isdiscussed above.

It should be noted that an inverse model of the above equation may bewritten as:h ^(T) x(t)=y(t)e ^(jφt) +v(t),

and could be used to derive the state model instead of the model derivedherein below.

The derivation of the state model, starting from equation (2), is nowdescribed below.

This estimator is designed to operate in the case of an equalizes, whichas discussed above means that there is a decision delay (n) related tothe acquired decisions. In accordance with the preferred embodiment ofthe present invention, the frequency error prediction is carried out foran input sample which goes to an equalizer at time (t+n), and thecorrection step is performed using all available information at time(t−1).

Therefore, as discussed above, there are two predictions: φ(t+n|t) andφ(t+1|t).

Also for the correction step, the sample related to the decision at timet is based on the frequency error estimate at time (t−n).

A state model of the frequency error can therefore be stated as:

$\begin{matrix}\left\{ {\begin{matrix}{{{{y(t)}{\mathbb{e}}^{{- {{j\alpha}{({t❘{t - n}})}}}t}} - {h^{T}{x(t)}}} = {{{{j\beta}\left( {t❘{t - 1}} \right)}{th}^{T}{x(t)}} + {w(t)}}} \\{{\overset{\Cap}{\varphi}\left( {{t + 1}❘t} \right)} = {{\overset{\Cap}{\varphi}\left( {t❘t} \right)} + {u(t)}}} \\{{\overset{\Cap}{\varphi}\left( {t❘{t - 1}} \right)} = {{a\left( t \middle| {t - n} \right)} + {\beta\left( {t❘{t - 1}} \right)}}}\end{matrix},} \right. & {{/3}/}\end{matrix}$where it can be noted that the last equation is additional to a normalKalman equation. It is used to define the bias between the non-linearand linear parts of equation 1. Therefore its derivation is close to anExtended Kalman Filter approach.

The term u(t) is a frequency noise term, which can be used to enhancethe tracking capabilities of the frequency estimator.

The frequency error estimator itself is now discussed.

The frequency error estimator procedure is based on thepredictor-corrector approach known from the Kalman filter. As thefrequency error component is divided into linear and non-linear parts,an additional step is carried out to study the relation between thoseparts.

First the relation between φ(t|t−1)=α(t|t−n)+β(t|t−1) is studied. It canbe noted that the linear approximation e^(jβt)≈(1+jβt) is only validwhen β is small and its quality gets worse as β gets larger. Thereforeby taking the expected values of equationE(φ(t|t−1))=E(α(t|t−n)+β(t|t−1)), it can be noted that E(β(t|t−1)) isminimised when φ(t|t−n)=α(t|t−n) (at time t−n.)

Using the information above a frequency error estimator (and corrector)can be derived. Note that two predictions of the frequency error arerequired. One may be used for rotating the samples to be used in theequalizer, and the other one may be used in the correct-on phase.

The necessary steps to be performed can therefore be defined as follows:

1. Find the frequency difference between different predictions andcorrect the frequency error estimate according the acquired decisionvector.β={circumflex over (φ)}(t|t−1)−α(t|t−n){circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T)x(t))2. Create new predictions:α(t+n|t)={circumflex over (φ)}(t|t){circumflex over (φ)}(t+1|t)={circumflex over (φ)}(t|t)

${P\left( {t + 1} \right)} = {\left( {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}} \right)^{- 1} + {E\left( {{u(t)}}^{2} \right)}}$3. Rotate the received samples with the prediction:y _(r)(t+n)=y(t+n)e ^(−jα(t+n|t)(t+n))

The y_(r) samples are then used in the equalizer.

In the initial phase, estimates for P(t_(initial)) and φ(t_(initial))are needed and the samples y_(r) are assumed to be rotated accordingly.

The simplification of the above equations is now discussed.

-   1. It can be noted that in case the u(t)=0, the variance of the    frequency estimate for a time (t+n) can be used in the correction    step and the calculation can be combined

${P^{- 1}\left( {t + 1} \right)} = {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}}${circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+P(t+1)δ_(w) ² x^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T) x(t))

-   2. If n is small, the β˜0 and the correction can be simplified    {circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P    ⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−h ^(T) x(t))

1. A method, comprising: receiving a signal at a time t, the signal including frequency error; removing from said signal an estimate of the frequency error in said signal, thereby generating a frequency corrected signal representing the signal received at time t; equalizing said received signal, wherein the equalizing introduces a delay of n samples, such that at time t an equalized output is generated representing a signal at time t−n; generating an estimate of a first component of the frequency error in the signal received at time t based on the equalized output representing the signal at time t−n; generating an estimate of a second component of the error in the signal received at time t based on an equalized output at time t−1 recalculating the first component in dependence on the frequency corrected received signal representing the signal at time t and recalculating the second component; and in dependence on the recalculated first component, estimating a first prediction of the frequency error for time t+n, wherein the first prediction of the estimated frequency error for time t+n is used to generate a frequency corrected signal for a signal received at time t+n and in dependence on the recalculated second component estimating a second prediction of the frequency.
 2. The method of claim 1, wherein the generating said estimate of the first component and recalculating the first component use the acquired decision vector: β={circumflex over (φ)}(t|t−1)−α(t|t−n) {circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P ⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T) x(t))
 3. The method of claim 1, wherein the estimating a value of the first component uses the predictions: α(t+n|t)={circumflex over (φ)}(t|t) {circumflex over (φ)}(t+1|t)={circumflex over (φ)}(t|t) ${P\left( {t + 1} \right)} = {\left( {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}} \right)^{- 1} + {E\left( {{u(t)}}^{2} \right)}}$
 4. The method of claim 1, wherein the removing from said signal said estimate of the frequency error in the received signal at time t+n uses the prediction: y _(r)(t+n)=y(t+n)e ^(−jα(t+n|t)(t+n))
 5. The method of claim 1, wherein the generating said estimate of the first component and recalculating the first component use the acquired decision vector: {circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+(δ_(w) ² P ⁻¹(t)+x ^(H)(t)h*h ^(T) x(t))⁻¹ x ^(H)(t)h*(y _(r)(t)−h ^(T) x(t))
 6. The method of claim 5, wherein the generating said estimate of the first component and recalculating the first component use the acquired decision vector: ${P^{- 1}\left( {t + 1} \right)} = {{P^{- 1}(t)} + {\frac{1}{\delta_{w}^{2}}{x^{H}(t)}h^{*}h^{T}{x(t)}}}$ {circumflex over (φ)}(t|t)={circumflex over (φ)}(t|t−1)+P(t+1)δ_(w) ² x ^(H)(t)h*(y _(r)(t)−(1+jβt)h ^(T) x(t)) when u(t)=0.
 7. The method of claim 1, wherein the removing the estimate of the frequency error in a received signal comprises rotating the received signal with a prediction of said error.
 8. Equalization circuitry, comprising: rotation means for rotating a signal received at a time t with a prediction of the frequency error in the received signal and generating a frequency corrected signal representing the signal received at time t; equalizer means having a delay of n samples and for equalizing the frequency corrected signal representing the signal received at time t, and for generating an equalized output at time t+n representing the signal at time t; frequency error calculator means for receiving the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and for generating a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator means is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer means output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n, wherein this estimate is used by the rotation means to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator means is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer means at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated.
 9. An element of a mobile communication system comprising: equalization circuitry comprising rotation means for rotating a signal received at a time t with a prediction of the frequency error in the received signal and generating a frequency corrected signal representing the signal received at time t: equalizer means having a delay of n samples and for equalizing the frequency corrected signal representing the signal received at time t, and for generating an equalized output at time t+n representing the signal at time t: frequency error calculator means for receiving the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and for generating a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator means is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer means output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n, wherein this estimate is used by the rotation means to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator means is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer means at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated.
 10. A base transceiver station of a mobile communication system comprising: equalization circuitry comprising rotation means for rotating a signal received at a time t with a prediction of the frequency error in the received signal and generating a frequency corrected signal representing the signal received at time t: equalizer means having a delay of n samples and for equalizing the frequency corrected signal representing the signal received at time t, and for generating an equalized output at time t+n representing the signal at time t; frequency error calculator means for receiving the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and for generating a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator means is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer means output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n, wherein this estimate is used by the rotation means to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator means is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer means at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated.
 11. Equalization circuitry, comprising: a rotation unit configured to rotate a signal received at a time t with a prediction of the frequency error in the received signal and to generate a frequency corrected signal representing the signal received at time t; an equalizer having a delay of n samples and configured to equalize the frequency corrected signal representing the signal received at time t, and to generate an equalized output at time t+n representing the signal at time t; frequency error calculator configured to receive the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and to generate a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n, wherein this estimate is used by the rotation unit to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated.
 12. An element of a mobile communication system comprising: equalization circuitry comprising a rotation unit configured to rotate a signal received at a time t with a prediction of the frequency error in the received signal and to generate a frequency corrected signal representing the signal received at time t: an equalizer having a delay of n samples and configured to equalize the frequency corrected signal representing the signal received at time t, and to generate an equalized output at time t+n representing the signal at time t;; frequency error calculator configured to receive the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and to generate a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n. wherein this estimate is used by the rotation unit to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated.
 13. A base transceiver station of a mobile communication system comprising: equalization circuitry comprising a rotation unit configured to rotate a signal received at a time t with a prediction of the frequency error in the received signal and to generate a frequency corrected signal representing the signal received at time t; an equalizer having a delay of n samples and configured to equalize the frequency corrected signal representing the signal received at time t, and to generate an equalized output at time t+n representing the signal at time t; frequency error calculator configured to receive the equalized output at time t+n and the frequency corrected signal representing the signal received at time t, and to generate a prediction of the frequency error in the signal at time t+n, wherein the frequency error calculator is configured to generate an estimate of a first component of the frequency error in the signal received at time t based on the equalizer output representing the signal at time t−n, to recalculate this first component in dependence on the frequency corrected signal representing the signal at time t, and in dependence on such recalculated component to estimate a value of the first component for the frequency error at time t+n, wherein this estimate is used by the rotation unit to generate a frequency corrected signal for the signal received at time t+n, wherein the frequency error calculator is further configured to generate an estimate of a second component of the frequency error in the received signal based on an output of the equalizer at time t−1, and wherein said second component is recalculated and, in dependence thereon, a value of the second component of the frequency error for time t+1 is estimated. 